1. Field of the Invention
This invention relates to a semiconductor switch having at least four adjacent semiconductive layers, such as a thyristor, and to a method of making the same.
2. Description of the Prior Art
In general, in semiconductor switches, the four adjacent semiconductive layers are arranged to be alternately of the opposite conductive type. A pair of main current-carrying electrodes (Anode and Cathode) are provided in low resistance contact with the end layers, and at least one control electrode (gate) is similarly connected to an intermediate layer. The two intermediate layers between the end layers are termed the base layers, and both end layers are called emitter layers.
A central P-N junction is formed between both of the base layers, and two outer P-N junctions are formed between each emitter layer and the adjacent intermediate base layer. THe outer P-N junctions are called the emitter junctions which are positioned on both sides of the center P-N junction. A pair of main electrodes is coupled to the emitter layers, and a gate electrode is coupled to at least one of the base layers.
The forward voltage for switching (hereinafter referred to as the switching voltage), is applied between both of the electrical terminals connected to the emitter layers. In a two terminal thyristor, when the switching voltage is forwardly applied, a high impedance state is presented until the applied voltage reaches a rated value (hereinafter referred to as the forward break-down voltage). If the applied voltage exceeds the rated value, not only does the center P-N junction break down, but it reverses in polarization and a very low impedance state is presented between the terminals.
In a thyristor with a gate electrode, the gate current is supplied between one of the base layers, and the emitter layer adjacent to that base layer, whereby the gate current changes the break-down voltage of the device.
In general, the gate current is supplied so as to switch the thyristor from its forward blocking state to its forward conducting state. In such thyristors, it has been found that the forward blocking voltage capability is decreased by increases in junction temperature and increases in the rate of change (dv/dt) of applied forward voltage.
In both cases, leakage current flows across the center P-N junction, which is reversely biased in the forward blocking state. The leakage current acts as the forward current for both of the emitter junctions. Therefore, significant amounts of minority carries are injected to the base layers from the emitter layers, whereby the current amplification is increased and the forward blocking voltage capability is decreased.
It is possible to correct this decrease of forward blocking voltage capability due to increasing injection, provided the leakage current flowing to the center junction is by-passed at one of the emitter P-N junctions as as to prevent the unwanted injection caused by the leakage current.
In view of the above, a structure has been proposed and widely used wherein a part of the electrode engaging the emitter layer with low resistance also engages the base layer adjacent to the emitter layer with low resistance. This is commonly called the "shorted emitter structure".
However, disadvantageous effects are produced in passing a large leakage current from the base layer directly to the outer electrode to prevent the injection of the minority carriers from the emitter layer. More particularly, in shorted emitter thyristors having a gate electrode, the gate triggering current is increased, and thus the gate input power is disadvantageously increased. Also in this type of thyristor, in voltage triggering by applying a voltage exceeding the break-down voltage with no gate current, the current required for the break-down (break-down current) is increased, whereby excess power is dissipated resulting in the possibility of a secondary break-down-like phenomenon as is found in a conventional transistor.
Moreover, in the shorted emitter thyristors, the rate of stored carrier flow in the vicinity of the shorted area of the active base region is reduced by spreading of the turn-on region at the initiation of the forward conducting state. This is caused by flowing out of large amounts of stored carriers from the base layer directly to the outer main electrode through the emitter shunt path. As a consequence, the spreading velocity of the turn-on region is reduced and the rate of change of the current (di/dt) is disadvantageously described.
These disadvantages can be slightly improved by increasing the sheet resistance of the base layer and limiting the leakage current passed to the emitter junction short circuit. However, the increase in the sheet resistance of the base layer is limited from the structural viewpoint, and it is consequently hard to completely eliminate the above-described difficulties.